The Role of the Enterohepatic Circulation of Bile Salts and Nuclear Hormone Receptors in the Regulation of Cholesterol Homeostasis: Bile Salts As Ligands for Nuclear Hormone Receptors

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ORIGINAL ARTICLE

The role of the enterohepatic circulation of bile salts and nuclear hormone receptors in the regulation of cholesterol homeostasis: Bile salts as ligands for nuclear hormone receptors

Richard N Redinger MD

RN Redinger. The role of the enterohepatic circulation of bile Le rôle de la circulation entérohépatique des salts and nuclear hormone receptors in the regulation of sels biliaires et des récepteurs de l’hormone cholesterol homeostasis: Bile salts as ligands for nuclear hormone receptors. Can J Gastroenterol 2003;17(4):265-271. nucléaire dans la régulation de l’homéostasie cholestérolique : Les sels biliaires comme The coordinated effect of lipid activated nuclear hormone receptors; ligands des récepteurs de l’hormone nucléaire liver X receptor (LXR), bound by oxysterol ligands and farnesoid X receptor (FXR), bound by bile acid ligands, act as genetic transcrip- L’effet coordonné des récepteurs nucléaires de l’hormone activée par les tion factors to cause feed-forward cholesterol catabolism to bile acids lipides, du récepteur X hépatique (RXH), lié par les ligands d’oxystérol et and feedback repression of bile acid synthesis, respectively. It is the le récepteur X famésoïde (RXF), liés par les ligands acidobiliaires, agit coordinated action of LXR and FXR, each dimerized to retinoid X comme des facteurs de transcription génétique afin de provoquer, respec- receptor, that signal nuclear DNA response elements to encode pro- tivement, un catabolisme cholestérolique non récurrent des sels biliaires et teins that prevent excessive cholesterol accumulation and bile salt une répression à rétroaction de la synthèse de l’acide biliaire. C’est l’action toxicity, respectively. LXR helps prevent hypercholesterolemia by coordonnée du RXH et du RXF, chacun étant dimérisé au récepteur X enhancing transporters for cholesterol efflux that enhance reverse rétinoïde, qui signale aux éléments de réponse de l’ADN nucléaire d’en- cholesterol transport, while FXR enhances intestinal reabsorption coder les protéines qui, respectivement, empêchent l’accumulation and preservation of bile salts by increasing the ileal bile acid binding cholestérolique excessive et la toxicité des sels biliaires. Le RXH con- protein. FXR also targets sodium taurocholate cotransport peptide tribue à prévenir l’hypercholestérolémie en accentuant l’efflux and bile salt export pump (protein) genes to limit bile salt uptake and cholestérolique des transporteurs, qui accroît le transport cholestérolique enhance export, respectively, which prevents bile salt toxicity. Other inversé, tandis que le RXF favorise la réabsorption intestinale et la préser- nuclear hormone receptors such as pregnan X receptor, which share vation des sels biliaires en augmentant la liaison de la protéine à l’acide biliaire iléal. Le RXF cible également le peptide de cotransport du tauro- the obligate partner, retinoid X receptor, and vitamin D receptor also cholate sodique et les gènes de la pompe (à protéines) d’exportation des function as bile acid sensors to signal detoxification by hydroxylation sels biliaires, respectivement, lesquels empêchent la toxicité des sels bi- of toxic bile acids. Pharmacologically targeted receptor agonists (or liaires. D’autres récepteurs nucléaires de l’hormone, tels que le récepteur antagonists) may be developed that alter cholesterol and bile salt X pregnan, qui partage le partenaire strict, le récepteur X rétinoïde, et le concentrations by modulating nuclear hormone receptors and/or récepteur de la vitamine D, fonctionnent également comme des senseurs their coactivators or corepressors to positively affect cholesterol de l’acide biliaire afin de signaler la détoxification par l’hydroxylation des homeostasis and bile salt metabolism. It is the coordinated transcrip- acides biliaires toxiques. Des agonistes (ou antagonistes) récepteurs ciblés tion factor action of LXR, which responds to ligand binding of circu- par la pharmacologie peuvent être mis au point pour altérer les concen- lating oxysterols in both liver and peripheral tissues, and FXR trations de cholestérol et de sels biliaires en modulant les récepteurs de responding to bile salts within the enterohepatic circulation that l’hormone nucléaire, leurs coactivateurs ou leurs corépresseurs à avoir un make possible the regulation of cholesterol and bile acid homeostasis. effet positif sur l’homéostasie cholestérolique et le métabolisme des sels biliaires. C’est l’action coordonnée des facteurs de transcription du RXH Key Words: Bile acid; Cholesterol homeostasis; Nuclear hormone qui réagit à la liaison ligand des oxystérols circulants tant dans le foie que receptors dans les tissus périphériques, et du RXF qui réagit aux sels biliaires dans la circulation entérohépatique qui rend possible la régulation du cholestérol et l’homéostasie de l’acide biliaire.

t is well known that bile salts (note: bile acids become bile as other lipids such as monoglycerides, fatty acids and fat- Isalts at physiological cellular pH=7. In the present review, soluble vitamins for luminal digestion and consequent intes- the term bile acid is used to denote their state at the time of tinal absorption (1). The physiochemical mechanisms under- synthesis while the designation bile salt is used to reflect their lying these actions were worked out by seminal studies on chemical state in body solutions as sodium and potassium salts biliary lipids and intraluminal intestinal digestion, thus provid- of bile acids) have major biological effects related to their abil- ing a better understanding of the pathogenesis of gallstone dis- ity to solubilize cholesterol in bile and in the intestine, as well ease (2) and luminal gut lipid malabsorption (3). However, the

Department of Medicine, University of Louisville, Louisville, Kentucky, USA Correspondence and reprints: Dr Richard Redinger, Department of Medicine, University of Louisville, 530 South Jackson Street, Third floor, Louisville, Kentucky 40292 USA. Telephone 502-852-5241, fax 502-852-6233, e-mail [email protected] Received for publication June 10, 2002. Accepted December 19, 2002

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role of nuclear hormone receptors within the enterohepatic ostasis. Bile acids, the catabolic end product of cholesterol, are circulation of bile salts has not been fully appreciated, which much more hydrophilic than cholesterol and in fact act as relates to the molecular regulation of bile salt and cholesterol detergents by forming mixed micelles with cholesterol and oth- homeostasis that prevents intrahepatic bile salt toxicity and er lipids for their transport to and absorption from the intestine abnormal accumulation of cholesterol or their esters, respec- (2). Bile salts themselves can become toxic at high levels in tively. Recently, through major advances in research relating cells, so that their intracellular levels must also be regulated in to molecular signaling by ligand binding to nuclear hormone addition to that of cholesterol (15). receptors, it is now apparent that bile salts themselves act as Insight into the physiological complexity contributing to ligands to nuclear receptors and thereby play an increasingly the regulation of bile salt metabolism first became apparent important role in preserving cholesterol homeostasis in mam- with the description of an enterohepatic circulation of bile mals, including man (4-8). The present review is intended to salts, which anatomically defined the movement of bile salts help the clinical gastroenterologist/hepatologist understand by following their synthesis as bile acids in the liver; secretion the role of the molecular regulation of bile salts and cholesterol as bile salts into the intestine by the biliary tract; their avid homeostasis both within and complementary to the entero- reabsorption in the distal ileum; and consequent transport hepatic circulation of bile salts. Due to the limited scope of the back to the liver in portal blood (16). More than 95% of bile present review and space limitations, additional reviews are salt secretion is thereby preserved with the body, so that less listed in the bibliography for details of molecular structure and than 5% of the bile salt pool is lost by fecal elimination. other nuclear hormone receptors that control other aspects of Excessive bile salt loss also must be avoided, because bile salts lipid metabolism (9-11). have colonic effects on cyclic adenosine monophosphate- induced water secretion and will produce secretory diarrhea if EVOLUTIONARY IMPERATIVES FOR lost excessively into the colon (ie, bile salt enteropathy) (17). CHOLESTEROL METABOLISM This fine-tuned circulation of the bile salt pool of two to four The need for bile salts in human evolution traces back to the grams, which cycles several times a meal and a total of 10 or need for cells to synthesize cholesterol as well as to control its more times daily, produces a total effective feed-forward secre- excessive accumulation by metabolic degradation. As life pro- tion of 30 grams of bile salt per day. Thus, an economy of the gressed from non-nucleated prokaryotes to eurokaryotes, with enterohepatic circulation of bile salts is necessary for preserva- the development of multiorganelle structures involving com- tion of the bile salt pool (18). For this economy to be realized for the maintenance of a constant bile salt pool, feedback con- plex membrane functions, cholesterol was required as an essen- trol of bile acid synthesis is also necessary to limit hepatic syn- tial membrane constituent and substrate for hormone thesis only to those bile salts that are lost via fecal elimination synthesis. However, cellular control of sterol metabolism was (Figure 1) (16). Early physiological experiments carried out by necessary and required nuclear (genetic) regulation of diverse Redinger et al (16,18,19) revealed feedback regulation of syn- proteins functioning as enzymes, transporters and intracellular thesis including that of individual bile acids in primate models. protein binders that regulate cholesterol metabolism. Nuclear Enzymatic regulation of cholesterol 7-alpha hydroxylase receptors therefore evolved to orchestrate cholesterol home- (CYP7A1) synthesis by bile acids was first described by ostasis including its catabolism to bile acids (5,6,12,13). In Mosbach’s laboratory in 1970 (20). However, the details of addition, to the problem that cells had with degradation of the molecular nuclear regulation were only recently determined cholesterol sterol ring, they also had to cope with excessive from more recent elegant studies that have revealed the molec- cholesterol availability. For example, enhanced dietary choles- ular controls of enzyme regulation of feed-forward activation terol that became available for carnivorous mammals placed (12) and feedback repression of bile acid synthesis (21) that additional stresses on their cells so that sophisticated cellular regulates cholesterol homeostasis. adaptations were necessary to protect against excessive cholesterol accumulation. MOLECULAR CONTROL OF CHOLESTEROL HOMEOSTASIS CELLULAR ADAPTATIONS DURING The first descriptions of molecular regulation of intracellular EVOLUTIONARY DEVELOPMENT cholesterol levels came from the laboratory of Brown and Because eukaryotic cells could not degrade the cholesterol Goldstein (22), who found that membrane bound proteins sterol ring, they developed at least 14 multistep enzymatic reac- called sterol regulating element binding proteins residing in tions to degrade cholesterol to bile acids to maintain normal the nuclear envelope and endoplasmic reticulum were activat- intracellular cholesterol levels within narrowly defined physio- ed by declining levels of cholesterol or their metabolites (ie, logical levels (14). To create cholesterol homeostasis (ie, to oxysterols), so that by cleavage of these sterol regulating ele- balance cholesterol input with output), cholesterol had to be ment binding proteins, transcription factors were made avail- catabolized to bile acids by both multiple hydroxylation steps able to target the nucleus and transactivate sterol responsive and side chain shortening to accomplish the conversion of 27 genes (22,23). These genes in turn encoded key enzymes carbon cholesterol molecules to 24 carbon bile acid molecules including but not limited to 3-hydroxy-3-methylglutaryl coen- (14). This catabolism accounts for 50% of the conversion of zyme A reductase to enhance cholesterol synthesis. The sec- cholesterol derived from dietary input, intestinal absorption, ond major description of transcription regulators of cholesterol storage and synthesis. Because relatively small amounts of cho- homeostasis occurred with the discovery that nuclear hormone lesterol are converted to hormones (13), the remainder of cho- receptors acting as transcription factors regulate cholesterol lesterol output, apart from its catabolism to bile acids, occurs in metabolism in an opposing fashion. These receptors exist with- large part by fecal elimination of unabsorbed dietary, biliary in a superfamily of ligand-activated nuclear hormone tran- and effluxed cellular cholesterol to maintain cholesterol home- scription factors that regulate enzymatic expression of cellular

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Bile salts as ligands for nuclear hormone receptors

Figure 2) The nuclear hormone receptor, liver X receptor (LXR), activates the target gene cytochrome P450 cholesterol 7-alpha hydroxylase (CYP7A1) to increase bile salt synthesis from cholesterol (C). It increases the target genes for the ATP-binding cassette protein (ABC)- Figure 1) Cholesterol (C) synthesis and its relationship to the circula- A1 transporter to increase C efflux from peripheral cells for pickup of C tion of bile salts (BS) is shown. Hepatic C is derived from newly syn- by high density lipoprotein (HDL) and enhance reverse C transport to thesized cholesterol, high density lipoprotein (HDL) and low density liver. LXR transactivation of apolipoprotein E and cholesterol ester lipoprotein (LDL), diet via chylomicron remnants, and storage sources. transfer protein (CETP) genes and farnesoid X receptor (FXR) target- The enterohepatic circulation of BS begins with feed-forward BS secre- ed genes for apolipoprotein C-11 and phospholipid transfer protein tion after catabolism of hepatic cholesterol to bile acids. BS fecal losses (PLTP) further enhance HDL-C transport. LXR also targets genes for equal bile acid synthesis because of avid bile acid ileal reabsorption and enterocyte transporters ABC-G5/G8 to enhance C efflux for fecal C feedback repression of bile acid synthesis. Fecal C losses are the result elimination from the intestine. FXR with small heterodimer receptor of unabsorbed biliary and dietary intestinal C and effluxed C from (SHP) activates target genes to repress CYP7A1 which regulates bile enterocytes to maintain C balance/homeostasis. Boxed symbols, ie, salt (BS) synthesis to levels needed to maintain bile salt pool size, and [LDLr]* refer to the LDL receptor*, HDL scavenger receptor (SR)- increases the target gene for ileal bile acid binding protein (IBABP) to B1, and chylomicron remnant (CMR) receptor in the liver for uptake enhance ileal bile salt absorption and preserve the bile salt pool. of C by the liver. For C balance/homeostasis, C input (synthesis and Concomitantly it decreases the hepatic target genes for sodium tauro- diet) must equal C output (fecal BS and C losses). IDL Intermediate cholate contransport peptide (NTCP) controlling bile acid uptake while density lipoprotein; Vit D Vitamin D; VLDL Very low density lipopro- increasing that for bile salt export pump (protein) (BSEP) for bile acid tein export to protect against excessive bile salt toxicity. LDL Low density lipoprotein

activity controlling many diverse functions such as embryonic development, cell differentiation and lipid homeostasis (24). Ligands or signaling molecules for these receptors are small receptors into six subfamilies (24). Subfamily I contains most lipophilic molecules within cells, which cross lipid cell mem- of the nuclear hormone receptors found within and associated branes, bind to nuclear receptors, and activate transcriptional with the enterohepatic circulation of bile salts that function to messages within nuclear DNA that encode targeted genes control cholesterol and, even more broadly, many other (Figure 2). As such, the superfamily consists of phylogenetical- aspects of lipid metabolism (25). The receptors in this subfam- ly related nuclear receptor proteins that share DNA ancestral ily include liver X receptor (LXR), farnesoid X receptor (FXR), domain structures common to both early eurokaryotes and pregnane X receptor (PXR), peroxisome proliferator activated advanced organisms including mammals (24). These receptors receptor (PPARγ), and vitamin D receptor (VDR) which are contain a well-conserved central DNA binding domain that further discussed in this review. Subfamily II contains retinoid allows classification and a moderately conserved ligand bind- X receptor (RXR), which dimerizes with and acts as an obli- ing domain that allows selective binding of specific ligands to gate partner to FXR, LXR and PXR (See Figure 3 for FXR/RXR nuclear hormone receptors. These in turn perform as transcrip- dimer illustration), while Subfamily III contains steroid recep- tion factors to activate DNA encoding of enzymes regulating tors for estrogen, androgen and progesterone receptors. Other homeostatic control of lipid metabolism including that of cho- subfamilies (IV, V and VI) contain orphan receptors, ie, recep- lesterol homeostasis (Figure 3). The commonality of these tors found without known ligands at the time of their discov- receptor DNA domains allows the classification of nuclear ery, which are also found in many tissues, including some

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pathways involves ligand binding to a heterodimer receptor complex, eg, FXR/RXR, (Figure 3), which signals a responsive element domain containing a specific DNA motif within nuclear DNA to transactivate transcription messages that encode target genes for the expression of protein enzymes. For further details of additional modifiers of nuclear hormone receptors, see the review by Aranda and Pascual (11).

LXR FUNCTIONS WITHIN THE ENTEROHEPATIC CIRCULATION Figure 3) An example of the heterodimer nuclear hormone receptor Oxysterol ligands, which are oxidized cholesterol metabolites, farnesoid X receptor (FXR) along with the retinoid X receptor (RXR) bind to the LXR, a specialized dimerized receptor that also obligate partner is shown with their attached bile salt (BS) and 9 cis needs to be complexed to an obligate partner RXR to function. retinoic acid ligands (9CisRA). Ligand binding domains (LBD) and The latter receptor is itself a common obligate partner to oth- DNA binding domains (DBD) exist within these receptors, which inter- er members of Subfamily II noted earlier (24,39). The het- act with a nuclear response element with specific DNA motif to initiate erodimer receptor, LXR/RXR, while originally found in the transcription activity. Liver receptor homolog-1 (LRH-1), another liver, also exists in diverse tissue such as brain, macrophages nuclear receptor is a competence factor of LRH-1 {correct?}. The small heterodimer partner (SHP) is also activated by FXR. SHP without a and scavenger cells. In the liver, following ligand binding, it DNA binding site is unable to bind to the nuclear DNA motif but by signals sequential genetic transcription messages that encode inhibiting LRH-1, it indirectly represses cytochrome P450 cholesterol the target genes for the expression of CYP7A1. CYP7A1 up- 7-alpha hydroxylase (CYP7A1). Nuclear hormone receptors act as regulates bile acid synthesis from cholesterol by feed-forward transcription factors and with coordinated receptor coactivators genetic induction (encoding) of new bile acid synthesis. LXR (LRH-1) and corepressors (SHP) encode feedback repression of bile acid also targets genes in enterocytes to cause cholesterol efflux by synthesis transporter proteins ATP-binding cassette proteins (ABC)- A1, ABC-G5 and G8 (34,35,40,41). In this fashion, additional cholesterol may be eliminated by fecal cholesterol excretion within the enterohepatic circulation of bile salts, such as small (Figures 1 and 2) (34-37). Mutations of genes encoding ABC- heterodimer receptor (SHP) (discussed further in this review). G5 and ABC-G8 transporters result in sitosterolemia as they Screening strategies had to be developed to identify ligands are unable to efflux plant sterol sitosterol which then accumu- from known biological compounds for these orphan receptors lates in serum (42). so as to realize their physiological functions (12). Effective ligands for nuclear hormone receptor activation were consequently found to be small lipophilic hormone-like molecules NUCLEAR HORMONE RECEPTOR FUNCTIONS obtained from tissue extraction or synthetically prepared com- THAT ARE COMPLEMENTARY TO THE pounds. As many as 45 nuclear hormone receptors have now ENTEROHEPATIC CIRCULATION been discovered in the human genome, but details of their Targeted gene regulation activated by LXR receptors also exists molecular functions (eg, transcription regulation, coactiva- in peripheral tissues outside that of the enterohepatic circulation, corepression) are not completely understood (26-28). tion of bile salts to cause mobilization of cholesterol for return Receptor agonists that can substitute for these physiological to the liver and further metabolism of cholesterol (35). ligands have been found that can activate these transcription Reverse cholesterol transport itself is a consequence of multi- factors (5). Similarly, antagonists are being identified that ple nuclear hormone receptor actions which, as transcription interfere with normal receptor function (29). Table 1 contains factors, regulate lipid components within lipoproteins for a summary and characterization of such known receptor mod- reverse cholesterol transport (35,43). FXR signals enhanced ulators (30-35). Nuclear hormone receptors are also capable of apolipoprotein C-11 transcription (43) and phospholipid being activated or inhibited by molecules other than their lig- transfer protein (44-46), while LXR enhances cholesterol ands such as components of signal transduction pathways exchange transfer proteins (47), which facilitate cholesterol including immunomodulators (ie, inflammatory cytokines, G- ester and phospholipid transfer or exchange to high density proteins and kinases with their phosphorylation pathways) lipoprotein (HDL) from other lipoproteins for consequent (36). Lipid activated nuclear hormone receptors, their ligands, transport of HDL cholesterol back to the liver. Further gene and target genes involved in cholesterol and bile salt home- targets of LXR/RXR receptors include the ABC-A1 trans- ostasis have recently been discovered to be abundant in tissues porter that results in efflux of cholesterol out of macrophages within the territory of the enterohepatic circulation of bile and fibroblasts (35,40). In these peripheral tissues, effluxed salts (12) (Figures 1 and 2). These exist within the large sub- cholesterol is taken up within their vascular beds by small family that includes the non-steroidal nuclear hormone recep- naive HDL4 particles, which aided by apolipoproteins E (48) tors, LXR and FXR, which are two major receptors involved in and C11 (43), become larger mature HDL2 particles by virtue the regulation of cholesterol catabolism and elimination (5-7). of increased phospholipid and cholesterol content from phos- Oxysterol intermediates of cholesterol catabolism (37) as well pholipid transfer protein and cholesterol exchange transfer as bile salts (38), the final catabolic end products of cholesterol proteins activity as well as by uptake and esterification of cho- catabolism, have now been identified as ligands for the signal lesterol by lecithin cholesterol acyl transferase. Furthermore, pathways of transcription factor expression and consequent other nuclear hormones that are involved in lipid metabolism activation of enzyme induction (37) or repression, respectively such as PPARγ are also able to enhance the expression of LXR (38). The sequence of molecular events in these molecular target gene action on ABC-A1 transporters for cholesterol

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Bile salts as ligands for nuclear hormone receptors

TABLE 1 Nuclear hormone receptor modulators Nuclear receptor Agonist Mechanism Antagonist Reference LXR – NR1H3 1. Endogenous ligands Oxysterols Ligand binding Bile salts 4,5,12 Target Genes: 27-OH-Chol ↑ CYP7A1 ↓ CYP7A1 22-R-OH-Chol ↑ ABC-A1, G5/G8 34,35 20-S-OH-Chol ↑ CETP 47 25-S-25-Epoxy-Chol ↑ apo E 5,48

2. Pharmacological Cholestyramine ↑ CYP7A1 (BS sequestrant) None identified 5

3. Synthetic (Experimental) All selective nonsteroidal 33 GW3965 ↑ ABC-A1 34 T1317 (70901317) ↑ ABC-A1 36 LG268 Retinoid (↑ ABC-A1) 30,34 GW4064 ↑ BSEP 32 Acetyl Podocarpic Dimer ↑ ABC-A1

FXR - NR1H4 1. Endogenous ligands Bile acids CDCA>LCA>DCA Ligand binding Target Genes: ↓ CYP7A1, ↑ apo C-11, ↑ PLTP 43,44 ↓ NTCP, ↑ BSEP, ↑ IBABP 54,56,57

2. Pharmacological Ursodeoxycholic acid Activates PXR, ↑ CYP3A 30,31 Replaces hydrophobic BS Guggelsterone 29 Pregnanedione 3. Synthetic (Experimental) All nonsteroidal GW4064 Isoxazole GW9047 (Benzoic acid derivatives) TTNPB ↓ CYP7A1 30,31,34 LG268 Rexinoid (↓ CYP7A1) 36

In addition, please see reference 5 for a detailed review of agonist and antagonist actions. ABC A-1,G5/G8 ATP-binding cassette proteins A-1, G5 and G8; BS Bile salt; BSEP Bile salt export pump (protein); CDCA Chenodeoxycholic acid; CETP Cholesterol ester transfer protein; CYP3A Cholesterol 6α hydroxylase; CYP7A1 Cytochrome P450 cholesterol 7-alpha hydroxylase; DCA Deoxycholic acid; FXR Farnesoid X receptor – NR1H4; IBABP Ileal bile acid binding protein; LCA Lithocholic acid; LXR Liver X receptor – NR1H3; NTCP Sodium taurocholate cotransport peptide; PLTP Phospholipid transfer protein; PXR Pregnane X receptor – NR1I2

efflux (25). In this manner, cholesterol esters are transported deorphanized nuclear receptor (49-51), which in so doing for return to the liver for uptake through the scavenger recep- allows FXR to indirectly repress bile acid synthesis by feedback tor B1 and consequent hepatic catabolism to bile acids (Figure control of CYP7A1 (Figures 2 and 3). An alternative explana- 1). tion is that down-regulation of CYP7A1 involves jun N-terminal kinase, which after activation by bile salts, enhances SHP FXR FUNCTIONS WITHIN THE transcription (52). It is hypothesized that SHP, by forming an ENTEROHEPATIC CIRCULATION inhibitory heterodimer complex with liver receptor homolog-1, Bile salt intracellular concentrations also need to be controlled also autoregulates itself (38,49). Furthermore, dimerized FXR within tissues of the enterohepatic circulation of bile salts to also up-regulates the ileal bile acid binding protein to enhance avoid intracellular bile salt toxicity. Another receptor, the FXR, ileal reabsorption of bile salts (53,54). In this fashion, FXR is named after the lipid farnesol, which was its first discovered mediates ileal bile acid binding protein gene expression to ligand (5). It is also dimerized to RXR (ie, FXR/RXR), and increase ileal bile acid absorption thereby preserving the exist- when activated by natural bile salt ligands initiates feedback ing bile salt pool as it concomitantly represses hepatic bile acid control of bile acid synthesis (38,49). Chenodeoxycholic acid, synthesis (54,55). FXR additionally decreases hepatic bile salt lithocholic acid and deoxycholic acid are the most potent bile uptake by down-regulating the hepatic cell basolateral sodium salt ligands. FXR, however, requires the assistance of another taurocholate polypeptide transporter (56). Simultaneously, it orphan receptor, the SHP, which is a heterodimer with no up-regulates hepatic bile salt canalicular transport into bile by DNA binding domain (49). It is speculated that SHP inhibits gene targeted protein induction of the bile salt export pump, the competence factor, liver receptor homolog-1, yet another thereby enhancing bile salt transport out of the liver (57). In

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this fashion, an efficient bile salt pool is preserved while exces- cholestasis, or other cholesterol/bile salt associated disorders sive bile salt levels are avoided in hepatocytes, all regulated by such as gallstone disease. coordinated action of the nuclear hormone receptor FXR at liv- It is the coordinated signaling and targeted gene action of er and intestinal sites of action (51,54-57). Other nuclear hor- the nuclear hormone receptor FXR within the enterohepatic mone receptors such as steroid and xenobiotic receptor/PXR tissues responding to ligand binding by circulating bile salts target the gene for induction of cholesterol 6-alpha hydroxylase combined with LXR binding by oxysterols and its targeted (CYP3A), which enhances 6-alpha hydroxylation of toxic bile gene actions in peripheral cells, liver and intestine, respective- acids such as lithocholic acid (58,59). Hydroxylation increases ly, that achieve cholesterol homeostasis by enhancing reverse hydrophilicity resulting in increased urinary and fecal litho- cholesterol transport, cholesterol catabolism to bile acids and cholic acid (LCA) losses (58-60). enterocyte cholesterol efflux for fecal elimination. FXR targeted gene action assisted by other bile acid sensors, PXR and SUMMARY AND FUTURE DIRECTIONS VDR, in the liver and intestine prevent bile salt toxicity and The coordinated effects of nuclear hormone receptors bound preservation of the bile salt pool to affect bile salt homeostasis by oxysterol ligands signal genetic transcription factors to within the enterohepatic circulation of bile salts. cause feed-forward control of cholesterol catabolism to bile acids, while bile salts act as ligands for feedback repression of REFERENCE bile acid synthesis. Consequently, intrahepatic toxicity (due to 1. Hofmann AF. Enterohepatic circulation of bile acids. 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